It is desirable to provide a plastic closure with a press-on screw-off structure that is adapted for production, for easy use, and adapted for vacuum capping operation for liquids such as apple juice that are warm when capped, the temperatures being moderately high such as in the range of 180.degree.-205.degree. F. It is also desirable to have a simple closure that would permit the release of internal pressure in the event of product spoilage and one that can be removed very easily with relatively small amount of torque.
It has heretofore been proposed, for example, in U.S. Pat. No. 3,371,813 (Owen et al) to provide a soft readily deformable gasket material inside the skirt of a metal closure, the gasket material being said to deform readily and flow around the threads of a container to form cooperating thread grooves in the gasket material. The gasket material is soft, deformable and a material that sets to retain the grooves. This formation of well defined cooperating set grooves for threads in the gasket material is said to provide purchase or leverage on the threads to permit opening of the container by twisting the cap. The U.S. Pat. No. 3,371,813 suggests that the resilient deformable gasket material forming the ribs be at least about 20% and generally about 30-70% of the total circumferential surface area of the gasket skirt to provide increased package security and greater cam off force against the continuous threads.
U.S. Pat. No. 4,000,825 (Westfall) for a press-on twistoff metal closure for oxygen sensitive products describes a top liner having spaced-apart vertical ribs inside the skirt of the shell of the closure. The liner is formed preferably from a foamed plastisol material that deforms and cold flows around the threads of the neck of a container to form a series of thread-cooperating cavities in the foamed liner that sets to aid in removal of the closure using a twist-off motion. The patent shows a top seal in addition to sealing with deformation of the ribs.
In U.S. Pat. No. 3,270,904 (Foster et al) there is a press-on turn-off metal cap, the cap having a gasket material at the top and around the bottom of the skirt, the gasket material in the skirt engaging the threads of the container. The gasket material is described as plastomeric and takes a conformation to form a hermetic seal cooperating with the thread means of the container finish. The plastomeric material hardens to form rigid thread grooves therein for the camming operation with the raised threads of the neck of the container.
U.S. Pat. No. 3,606,062 (Frisch et al) discloses a crimp-on twist-off metal crown cap with top and corner seals. There are deformable metal flutes in the metal shell to engage the threads of the container.
U.S. Pat. No. 3,448,881 (Zipper) discloses a metal closure with a gasket for engaging the threads of a container, the metal closure having means to prevent an increase in cap removal torque including a ridge on the underside of the cap to prevent settling of the cap due to gradual deformation of the gasket material. Grooves are formed in the gasket by the threads of the container when the metal cap is applied to the threads.
In general, in the above patents that describe metal caps with soft deformable gaskets that set, there is substantial contact between the deformable gasket ribs and the container threads such as contact of the entire rib or as, for instance, in the Owens et al patent, the ribs are shaped for substantial contact with the threads and comprise 20% up to 70% of the gasket area. The forcing of the gasket ribs against the threads causes the soft deformable gasket ribs to form grooves, the material thereafter setting to retain the grooves.
It is an object of the present invention to provide a closure for a container in which there is a one-piece structure so constructed and arranged that ribs on the skirt slightly flex and bend around the threads of the container to form shallow indentations for easy twist-off of the cap requiring less torque than closures of the prior art in which the prior art ribs are made of relatively soft deformable gasket materials.
It is an object of the present invention to provide a plastic closure for a container having generally helical threads on the neck finish, the closure comprising an easily manufactured one-piece cap shell of thermoplastic material including the top wall and a peripheral skirt, the skirt having means thereon for engaging threads of the neck of a container, the means including a plurality of spaced-apart, flexible generally vertical thermoplastic ribs integral with the skirt, each rib being so constructed and arranged that it has sufficient resistance to cold flow that the rib only slightly flexes and bends around the thread to form a slight indentation on the rib when the rib is forced into contact with the thread, the slight indentation being sufficient to provide purchase on the threads for very easy removal of the closure.
It is an object of the present invention to provide a cap structure that is easy to manufacture and useful in a vacuum capping operation, the cap being of a vacuum type push-on twist-off cap which provides greater tolerance to wet conditions which generally inhibit tightening screw caps, permits release of internal pressure in the event of spoilage, and provides just enough purchase on the threads by means of hard, flexible thermoplastic ribs to provide enough torque to remove the cap by use of far less torque than is required by caps in the prior art.